Efficient tool for making tree-like shapes
Skin Modifier


add a new mesh object, go to edit mode, delete all vertices
add Skin modifier
in edit mode with Ctrl mouse click create a new vertex
now you can start draw branch by another click with Ctrl or press E (extrude)
change thickness of vertex by cmd+A on mac (Ctrl+A on pc?)




Notes: you can change size (thickness) also 


from 3Dview > Properties panel (N) > Vertex Data > Radius X/Y
or check or reasign shortcut in User Pref >  Keymap > search Skin Resize




Grease Pencil

You can use Annotation or Grease Pencil object to draw branch and convert to mesh and use Skin modifier. But it is probably more work. Like with Annotate tool ** you will have to Convert to GP, than Convert GP to Curve, than Convert to Mesh. Using default Annotate tool generates smooth but vertex intensive line that doesn't fits to Skin modifier later (you would have to use Simplify add-on in step Convert GP to Curve ...